In designing antenna structures, it is desirable to provide appropriate gain, bandwidth, beamwidth, sidelobe level, radiation efficiency, aperture efficiency, radar cross-section (RCS), radiation resistance and other electrical characteristics. It is also desirable for these structures to be lightweight, simple in design, inexpensive and unobtrusive, since an antenna is often required to be mounted upon or secured to a supporting structure or vehicle, such as high velocity aircraft, missiles, rockets or even artillery projectiles, which cannot tolerate excessive deviations from aerodynamic shapes. It is also sometimes desirable to hide the antenna structure so that its presence is not readily apparent for aesthetic and/or security purposes. Accordingly, it is desirable that an antenna be physically small in volume and not protrude on the external side of a mounting surface, such as an aircraft skin, while yet still exhibiting all the requisite electrical characteristics.
One type of antenna that has been successfully used for broadband conformal applications is the Doorstop™ antenna. The Doorstop™ antenna belongs to a class of antennas known as traveling wave antennas. Examples of other traveling wave antennas are polyrod, helix, long-wires, Yagi-Uda, log-periodic, slots and holes in waveguides, and horns. Antennas of this type have very nearly uniform current and voltage amplitude along their length. This characteristic is achieved by carefully transitioning from the element feed and properly terminating the antenna structure so that reflections are minimized. An example of a Doorstop™ antenna is found in U.S. Pat. No. 4,931,808, assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated herein by reference.
A Doorstop™ antenna generally comprises a feed placed over a dielectric wedge, a groundplane supporting or adjacent to the dielectric wedge, and a cover or radome. The Doorstop™ antenna has two principal regions of radiation that affect patterns: the feed region and the lens region. The size and shape of these two regions generally control bandwidth and pattern performance.
In a typical Doorstop™ antenna, the measured voltage standing wave ratio (VSWR) improves with increasing frequency. At reduced frequencies the Doorstop™ element is electrically too short and functions more like a bent monopole antenna. The low frequency limit for the Doorstop™ element is set by the electrical depth of the element. More particularly, the maximum wedge depth and wedge dielectric constant determine the lowest frequency of operation. Once the physical depth and dielectric constant of the wedge are established, the lens to feed length ratio of the basic Doorstop™ configuration determines the pattern performance. At low frequencies, the pattern tends to look very uniform and nearly omni-directional, while at high frequencies the pattern becomes quite directional or end-fired. Additionally, at high frequencies the pattern develops a characteristic null at the zenith that moves forward toward the horizon as the frequency increases. For certain applications and greater operating bandwidths, this characteristic pattern performance is undesirable.
Within about a 3 to 1 operating bandwidth, the pattern characteristic can be controlled by adjusting the lens to feed length ratio of the antenna. As the frequency increases above the 3 to 1 ratio, the lens becomes electrically long, producing field components that either support or interfere with the radiation from the feed region. This leads to the creation of nulls in the forward portion of the farfield elevation plane pattern.
Other aspects of the typical Doorstop™ antenna that degrade performance include the use of an unsupported (not grounded) microstrip line near the coax feed, which adversely affects the element impedance match. Also, the coaxial pin typically used to interconnect the feed to a transmission line and the microstrip line are sources of radiation, that can degrade pattern performance by creating pattern nulls at certain angles. In addition, trapped energy in the dielectric wedge results in large impedance variation at low frequencies. As still another disadvantageous feature, because the element feed of a typical Doorstop™ antenna is on the surface of the device, it is exposed to improper handling and high temperatures that cause variation in radio-frequency (RF) performance.